Hematopoietic stem cell cytokines and fibroblast growth factor-2 stimulate human endothelial cell-pericyte tube co-assembly in 3D fibrin matrices under serum-free defined conditions

Selective mural cell recruitment of pericytes to networks of assembling endothelial cell-lined tubes

Mural cells are critically important for the development, maturation, and maintenance of the blood vasculature. Pericytes are predominantly observed in capillaries and venules, while vascular smooth muscle cells (VSMCs) are found in arterioles, arteries, and veins. In this study, we have investigated functional differences between human pericytes and human coronary artery smooth muscle cells (CASMCs) as a model VSMC type. We compared the ability of these two mural cells to invade three-dimensional (3D) collagen matrices, recruit to developing human endothelial cell (EC)-lined tubes in 3D matrices and induce vascular basement membrane matrix assembly around these tubes. Here, we show that pericytes selectively invade, recruit, and induce basement membrane deposition on EC tubes under defined conditions, while CASMCs fail to respond equivalently. Pericytes dramatically invade 3D collagen matrices in response to the EC-derived factors, platelet-derived growth factor (PDGF)-BB, PDGF-DD, and endothelin-1, while minimal invasion occurs with CASMCs. Furthermore, pericytes recruit to EC tube networks, and induce basement membrane deposition around assembling EC tubes (narrow and elongated tubes) when these cells are co-cultured. In contrast, CASMCs are markedly less able to perform these functions showing minimal recruitment, little to no basement membrane deposition, with wider and shorter tubes. Our new findings suggest that pericytes demonstrate much greater functional ability to invade 3D matrix environments, recruit to EC-lined tubes and induce vascular basement membrane matrix deposition in response to and in conjunction with ECs.

Defining the Functional Influence of Endothelial Cell-Expressed Oncogenic Activating Mutations on Vascular Morphogenesis and Capillary Assembly

This study sought to define key molecules and signals controlling major steps in vascular morphogenesis, and how these signals regulate pericyte recruitment and pericyte-induced basement membrane deposition. The morphogenic impact of endothelial cell (EC) expression of activating mutants of Kirsten rat sarcoma virus (kRas), mitogen-activated protein kinase 1 (Mek1), phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha (PIK3CA), Akt serine/threonine kinase 1 (Akt1), Ras homolog enriched in brain (Rheb) Janus kinase 2 (Jak2), or signal transducer and activator of transcription 3 (Stat3) expression versus controls was evaluated, along with EC signaling events, pharmacologic inhibitor assays, and siRNA suppression experiments. Primary stimulators of EC lumen formation included kRas, Akt1, and Mek1, whereas PIK3CA and Akt1 stimulated a specialized type of cystic lumen formation. In contrast, the key drivers of EC sprouting behavior were Jak2, Stat3, Mek1, PIK3CA, and mammalian target of rapamycin (mTor). These conclusions are further supported by pharmacologic inhibitor and siRNA suppression experiments. EC expression of active Akt1, kRas, and PIK3CA led to markedly dysregulated lumen formation coupled to strongly inhibited pericyte recruitment and basement membrane deposition. For example, activated Akt1 expression in ECs excessively stimulated lumen formation, decreased EC sprouting behavior, and showed minimal pericyte recruitment with reduced mRNA expression of platelet-derived growth factor-BB, platelet-derived growth factor-DD, and endothelin-1, critical EC-derived factors known to stimulate pericyte invasion. The study identified key signals controlling fundamental steps in capillary morphogenesis and maturation and provided mechanistic details on why EC activating mutations induced a capillary deficiency state with abnormal lumens, impaired pericyte recruitment, and basement deposition: predisposing stimuli for the development of vascular malformations.

Recovery of Therapeutically Ablated Engineered Blood-Vessel Networks on a Plug-and-Play Platform

Limiting the availability of key angiogenesis-promoting factors is a successful strategy to ablate tumor-supplying blood vessels or to reduce excessive vasculature in diabetic retinopathy. However, the efficacy of such anti-angiogenic therapies (AATs) varies with tumor type, and regrowth of vessels is observed upon termination of treatment. The ability to understand and develop AATs remains limited by a lack of robust in vitro systems for modeling the recovery of vascular networks. Here, complex 3D micro-capillary networks are engineered by sequentially seeding human bone marrow-derived mesenchymal stromal cells and human umbilical vein endothelial cells (ECs) on a previously established, synthetic plug-and-play hydrogel platform. In the tightly interconnected vascular networks that form this way, the two cell types share a basement membrane-like layer and can be maintained for several days of co-culture. Pre-formed networks degrade in the presence of bevacizumab. Upon treatment termination, vessel structures grow back to their original positions after replenishment with new ECs, which also integrate into unperturbed established networks. The data suggest that this plug-and-play platform enables the screening of drugs with blood-vessel inhibiting functions. It is believed that this platform could be of particular interest in studying resistance or recovery mechanisms to AAT treatment.

Elucidating the Morphogenic and Signaling Roles of Defined Growth Factors Controlling Human Endothelial Cell Lumen Formation Versus Sprouting Behavior

Five growth factors [ie, insulin, fibroblast growth factor-2 (FGF-2), stem cell factor, IL-3, and stromal-derived factor 1α] in combination are necessary for human endothelial cells (ECs) to undergo tube morphogenesis, a process requiring both lumen formation and sprouting behavior. This study investigated why these factors are required by subdividing the factors into 4 separate groups: insulin-only, insulin and FGF-2, no FGF-2 (all factors but without FGF-2), and all factors. The study found that the insulin-only condition failed to support EC morphogenesis or survival, the insulin and FGF-2 condition supported primarily EC lumen formation, and the no FGF-2 condition supported EC sprouting behavior. By comparison, the all-factors condition more strongly stimulated both EC lumen formation and sprouting behavior, and signaling analysis revealed prolonged stimulation of multiple promorphogenic signals coupled with inhibition of proregressive signals. Pharmacologic inhibition of Jak kinases more selectively blocked EC sprouting behavior, whereas inhibition of Raf, phosphatidylinositol 3-kinase, and Akt kinases showed selective blockade of lumen formation. Inhibition of Src family kinases and Notch led to increased sprouting coupled to decreased lumen formation, whereas inhibition of Pak, Mek, and mammalian target of rapamycin kinases blocked both sprouting and lumen formation. These findings reveal novel downstream biological and signaling activities of defined factors that are required for the assembly of human EC-lined capillary tube networks.

Extracellular Matrix Regulation of Vascular Morphogenesis, Maturation, and Stabilization

The extracellular matrix represents a critical regulator of tissue vascularization during embryonic development and postnatal life. In this perspective, we present key information and concepts that focus on how the extracellular matrix controls capillary assembly, maturation, and stabilization, and, in addition, contributes to tissue stability and health. In particular, we present and discuss mechanistic details underlying (1) the role of the extracellular matrix in controlling different steps of vascular morphogenesis, (2) the ability of endothelial cells (ECs) and pericytes to coassemble into elongated and narrow capillary EC-lined tubes with associated pericytes and basement membrane matrices, and (3) the identification of specific growth factor combinations ("factors") and peptides as well as coordinated "factor" and extracellular matrix receptor signaling pathways that are required to form stabilized capillary networks.

Endothelial k-RasV12 Expression Induces Capillary Deficiency Attributable to Marked Tube Network Expansion Coupled to Reduced Pericytes and Basement Membranes

OBJECTIVE

We sought to determine how endothelial cell (EC) expression of the activating k-Ras (kirsten rat sarcoma 2 viral oncogene homolog) mutation, k-RasV12, affects their ability to form lumens and tubes and interact with pericytes during capillary assembly Approach and Results: Using defined bioassays where human ECs undergo observable tubulogenesis, sprouting behavior, pericyte recruitment to EC-lined tubes, and pericyte-induced EC basement membrane deposition, we assessed the impact of EC k-RasV12 expression on these critical processes that are necessary for proper capillary network formation. This mutation, which is frequently seen in human ECs within brain arteriovenous malformations, was found to markedly accentuate EC lumen formation mechanisms, with strongly accelerated intracellular vacuole formation, vacuole fusion, and lumen expansion and with reduced sprouting behavior, leading to excessively widened tube networks compared with control ECs. These abnormal tubes demonstrate strong reductions in pericyte recruitment and pericyte-induced EC basement membranes compared with controls, with deficiencies in fibronectin, collagen type IV, and perlecan deposition. Analyses of signaling during tube formation from these k-RasV12 ECs reveals strong enhancement of Src (Src proto-oncogene, non-receptor tyrosine kinase), Pak2 (P21 [RAC1 (Rac family small GTPase 1)] activated kinase 2), b-Raf (v-raf murine sarcoma viral oncogene homolog B1), Erk (extracellular signal-related kinase), and Akt (AK strain transforming) activation and increased expression of PKCε (protein kinase C epsilon), MT1-MMP (membrane-type 1 matrix metalloproteinase), acetylated tubulin and CDCP1 (CUB domain-containing protein 1; most are known EC lumen regulators). Pharmacological blockade of MT1-MMP, Src, Pak, Raf, Mek (mitogen-activated protein kinase) kinases, Cdc42 (cell division cycle 42)/Rac1, and Notch markedly interferes with lumen and tube formation from these ECs.

CONCLUSIONS

Overall, this novel work demonstrates that EC expression of k-RasV12 disrupts capillary assembly due to markedly excessive lumen formation coupled with strongly reduced pericyte recruitment and basement membrane deposition, which are critical pathogenic features predisposing the vasculature to develop arteriovenous malformations.

Proinflammatory mediators, TNFα, IFNγ, and thrombin, directly induce lymphatic capillary tube regression

In this work, we sought to investigate the direct effects of proinflammatory mediators on lymphatic endothelial cell (LEC) capillaries and whether they might induce regression. Our laboratory has developed novel in-vitro, serum-free, lymphatic tubulogenesis assay models whereby human LEC tube networks readily form in either three-dimensional collagen or fibrin matrices. These systems were initially conceptualized in the hopes of better understanding the influence of proinflammatory mediators on LEC capillaries. In this work, we have screened and identified proinflammatory mediators that cause regression of LEC tube networks, the most potent of which is TNFα (tumor necrosis factor alpha), followed by IFNγ (interferon gamma) and thrombin. When these mediators were combined, even greater and more rapid lymphatic capillary regression occurred. Surprisingly, IL-1β (interleukin-1 beta), one of the most potent and pathologic cytokines known, had no regressive effect on these tube networks. Finally, we identified new pharmacological drug combinations capable of rescuing LEC capillaries from regression in response to the potent combination of TNFα, IFNγ, and thrombin. We speculate that protecting lymphatic capillaries from regression may be an important step toward mitigating a wide variety of acute and chronic disease states, as lymphatics are believed to clear both proinflammatory cells and mediators from inflamed and damaged tissue beds. Overall, these studies identify key proinflammatory mediators, including TNFα, IFNγ, and thrombin, that induce regression of LEC tube networks, as well as identify potential therapeutic agents to diminish LEC capillary regression responses.

Molecular basis for pericyte-induced capillary tube network assembly and maturation

Here we address the functional importance and role of pericytes in capillary tube network assembly, an essential process that is required for vascularized tissue development, maintenance, and health. Healthy capillaries may be directly capable of suppressing human disease. Considerable advances have occurred in our understanding of the molecular and signaling requirements controlling EC lumen and tube formation in 3D extracellular matrices. A combination of SCF, IL-3, SDF-1α, FGF-2 and insulin ("Factors") in conjunction with integrin- and MT1-MMP-induced signaling are required for EC sprouting behavior and tube formation under serum-free defined conditions. Pericyte recruitment to the abluminal EC tube surface results in elongated and narrow tube diameters and deposition of the vascular basement membrane. In contrast, EC tubes in the absence of pericytes continue to widen and shorten over time and fail to deposit basement membranes. Pericyte invasion, recruitment and proliferation in 3D matrices requires the presence of ECs. A detailed analysis identified that EC-derived PDGF-BB, PDGF-DD, ET-1, HB-EGF, and TGFβ1 are necessary for pericyte recruitment, proliferation, and basement membrane deposition. Blockade of these individual factors causes significant pericyte inhibition, but combined blockade profoundly interferes with these events, resulting in markedly widened EC tubes without basement membranes, like when pericytes are absent.

Heterotypic Multicellular Spheroids as Experimental and Preclinical Models of Sprouting Angiogenesis

Sprouting angiogenesis is the common response of live tissues to physiological and pathological angiogenic stimuli. Its accurate evaluation is of utmost importance for basic research and practical medicine and pharmacology and requires adequate experimental models. A variety of assays for angiogenesis were developed, none of them perfect. In vitro approaches are generally less physiologically relevant due to the omission of essential components regulating the process. However, only in vitro models can be entirely non-xenogeneic. The limitations of the in vitro angiogenesis assays can be partially overcome using 3D models mimicking tissue O2 and nutrient gradients, the influence of the extracellular matrix (ECM), and enabling cell-cell interactions. Here we present a review of the existing models of sprouting angiogenesis that are based on the use of endothelial cells (ECs) co-cultured with perivascular or other stromal cells. This approach provides an excellent in vitro platform for further decoding of the cellular and molecular mechanisms of sprouting angiogenesis under conditions close to the in vivo conditions, as well as for preclinical drug testing and preclinical research in tissue engineering and regenerative medicine.

Protective Effects of Collagen Tripeptides in Human Aortic Endothelial Cells by Restoring ROS-Induced Transcriptional Repression

Collagen tripeptide (CTP) is defined as a functional food material derived from collagenase digests of type I collagen and contains a high concentration of tripeptides with a Gly-X-Y sequence. CTP has several biological effects, including the acceleration of fracture healing, ameliorating osteoarthritis, and improving dryness and photoaging of the skin. Recently, an antiatherosclerotic effect of CTP has been reported, although its molecular mechanism is yet to be determined. In this study, we examined the effects of CTP on primary cultured human aortic endothelial cells (HAECs) under oxidative stress, because oxidative endothelial dysfunction is a trigger of atherosclerosis. DNA microarray and RT-qPCR analyses showed that CTP treatment recovered the downregulated expression of several genes, including the interleukin-3 receptor subunit alpha (IL3RA), which were suppressed by reactive oxygen species (ROS) treatment in HAECs. Furthermore, IL3RA knockdown significantly decreased the viability of HAECs compared with control cells. RT-qPCR analysis also showed that solute carrier 15 family peptide transporters, which are involved in CTP absorption into cells, were expressed in HAECs at levels more than comparable to those of a CTP-responsive human osteoblastic cell line. These results indicated that CTP exerts a protective effect for HAECs, at least in part, by regulating the recovery of ROS-induced transcriptional repression.

DNA Damage Baseline Predicts Resilience to Space Radiation and Radiotherapy

Deep space exploration will require real-time, minimally invasive monitoring of astronaut health to mitigate the potential health impairments caused by space radiation and microgravity. Genotoxic stress in humans can be monitored by quantifying the amount of DNA double-strand breaks (DSBs) in immune cells from a simple finger prick. In a cohort of 674 healthy donors, we show that the endogenous level of DSBs increases with age and with latent cytomegalovirus infection. To map the range of human responses to space radiation, we then study DSB induction and repair in immune cells from 319 healthy donors after the cells are exposed to galactic cosmic ray components and lymphocytes from 30 cancer patients after radiotherapy. Individuals with low baseline DSB have fewer clinical complications, enhanced DNA damage repair responses, and a functional dose-dependent cytokine response in healthy donor cells. This supports the use of DSB monitoring for health resilience in space.

Defining an Upstream VEGF (Vascular Endothelial Growth Factor) Priming Signature for Downstream Factor-Induced Endothelial Cell-Pericyte Tube Network Coassembly

OBJECTIVE

In this work, we have sought to define growth factor requirements and the signaling basis for different stages of human vascular morphogenesis and maturation. Approach and Results: Using a serum-free model of endothelial cell (EC) tube morphogenesis in 3-dimensional collagen matrices that depends on a 5 growth factor combination, SCF (stem cell factor), IL (interleukin)-3, SDF (stromal-derived factor)-1α, FGF (fibroblast growth factor)-2, and insulin (factors), we demonstrate that VEGF (vascular endothelial growth factor) pretreatment of ECs for 8 hours (ie, VEGF priming) leads to marked increases in the EC response to the factors which includes; EC tip cells, EC tubulogenesis, pericyte recruitment and proliferation, and basement membrane deposition. VEGF priming requires VEGFR2, and the effect of VEGFR2 is selective to the priming response and does not affect factor-dependent tubulogenesis in the absence of priming. Key molecule and signaling requirements for VEGF priming include RhoA, Rock1 (Rho-kinase), PKCα (protein kinase C α), and PKD2 (protein kinase D2). siRNA suppression or pharmacological blockade of these molecules and signaling pathways interfere with the ability of VEGF to act as an upstream primer of downstream factor-dependent EC tube formation as well as pericyte recruitment. VEGF priming was also associated with the formation of actin stress fibers, activation of focal adhesion components, upregulation of the EC factor receptors, c-Kit, IL-3Rα, and CXCR4 (C-X-C chemokine receptor type 4), and upregulation of EC-derived PDGF (platelet-derived growth factor)-BB, PDGF-DD, and HB-EGF (heparin-binding epidermal growth factor) which collectively affect pericyte recruitment and proliferation.

CONCLUSIONS

Overall, this study defines a signaling signature for a separable upstream VEGF priming step, which can activate ECs to respond to downstream factors that are necessary to form branching tube networks with associated mural cells.

Defining Endothelial Cell-Derived Factors That Promote Pericyte Recruitment and Capillary Network Assembly

OBJECTIVE

We sought to identify and investigate the functional role of the major endothelial cell (EC)-derived factors that control pericyte recruitment to EC tubes and pericyte-induced tube maturation during capillary network formation. Approach and Results: We identify PDGF (platelet-derived growth factor)-BB, PDGF-DD, ET (endothelin)-1, TGF (transforming growth factor)-β, and HB-EGF (heparin-binding epidermal growth factor), as the key individual and combined regulators of pericyte assembly around EC tubes. Using novel pericyte only assays, we demonstrate that PDGF-BB, PDGF-DD, and ET-1 are the primary direct drivers of pericyte invasion. Their addition to pericytes induces invasion as if ECs were present. In contrast, TGF-β and HB-EGF have minimal ability to directly stimulate pericyte invasion. In contrast, TGF-β1 can act as an upstream pericyte primer to stimulate invasion in response to PDGFs and ET-1. HB-EGF stimulates pericyte proliferation along with PDGFs and ET-1. Using EC-pericyte cocultures, individual, or combined blockade of these EC-derived factors, or their pericyte receptors, using neutralizing antibodies or chemical inhibitors, respectively, interferes with pericyte recruitment and proliferation. As individual factors, PDGF-BB and ET-1 have the strongest impact on these events. However, when the blocking reagents are combined to interfere with each of the above factors or their receptors, more dramatic and profound blockade of pericyte recruitment, proliferation, and pericyte-induced basement membrane deposition occurs. Under these conditions, ECs form tubes that become much wider and less elongated as if pericytes were absent.

CONCLUSIONS

Overall, these new studies define and characterize a functional role for key EC-derived factors controlling pericyte recruitment, proliferation, and pericyte-induced basement membrane deposition during capillary network assembly.

Robust and Scalable Angiogenesis Assay of Perfused 3D Human iPSC-Derived Endothelium for Anti-Angiogenic Drug Screening

To advance pre-clinical vascular drug research, in vitro assays are needed that closely mimic the process of angiogenesis in vivo. Such assays should combine physiological relevant culture conditions with robustness and scalability to enable drug screening. We developed a perfused 3D angiogenesis assay that includes endothelial cells (ECs) from induced pluripotent stem cells (iPSC) and assessed its performance and suitability for anti-angiogenic drug screening. Angiogenic sprouting was compared with primary ECs and showed that the microvessels from iPSC-EC exhibit similar sprouting behavior, including tip cell formation, directional sprouting and lumen formation. Inhibition with sunitinib, a clinically used vascular endothelial growth factor (VEGF) receptor type 2 inhibitor, and 3-(3-pyridinyl)-1-(4-pyridinyl)-2-propen-1-one (3PO), a transient glycolysis inhibitor, both significantly reduced the sprouting of both iPSC-ECs and primary ECs, supporting that both cell types show VEGF gradient-driven angiogenic sprouting. The assay performance was quantified for sunitinib, yielding a minimal signal window of 11 and Z-factor of at least 0.75, both meeting the criteria to be used as screening assay. In conclusion, we have developed a robust and scalable assay that includes physiological relevant culture conditions and is amenable to screening of anti-angiogenic compounds.

Human amnion-derived mesenchymal stem cells promote osteogenic differentiation of human bone marrow mesenchymal stem cells via H19/miR-675/APC axis

Bone volume inadequacy is an emerging clinical problem impairing the feasibility and longevity of dental implants. Human bone marrow mesenchymal stem cells (HBMSCs) have been widely used in bone remodeling and regeneration. This study examined the effect of long noncoding RNAs (lncRNAs)-H19 on the human amnion-derived mesenchymal stem cells (HAMSCs)-droved osteogenesis in HBMSCs. HAMSCs and HBMSCs were isolated from abandoned amniotic membrane samples and bone marrow. The coculture system was conducted using transwells, and H19 level was measured by quantitative real-time reverse transcription-polymerase chain reaction (RT-PCR). The mechanism was further verified. We here discovered that osteogenesis of HBMSCs was induced by HAMSCs, while H19 level in HAMSCs was increased during coculturing. H19 had no significant effect on the proliferative behaviors of HBMSCs, while its overexpression of H19 in HAMSCs led to the upregulated osteogenesis of HBMSCs in vivo and in vitro; whereas its knockdown reversed these effects. Mechanistically, H19 promoted miR-675 expression and contributed to the competitively bounding of miR-675 and Adenomatous polyposis coli (APC), thus significantly activating the Wnt/β-catenin pathway. The results suggested that HAMSCs promote osteogenic differentiation of HBMSCs via H19/miR-675/APC pathway, and supply a potential target for the therapeutic treatment of bone-destructive diseases.

Metabolic-vascular coupling in skeletal muscle: A potential role for capillary pericytes?

The matching of capillary blood flow to metabolic rate of the cells within organs and tissues is a critical microvascular function which ensures appropriate delivery of hormones and nutrients, and the removal of waste products. This relationship is particularly important in tissues where local metabolism, and hence capillary blood flow, must be regulated to avoid a mismatch between nutrient demand and supply that would compromise normal function. The consequences of a mismatch in microvascular blood flow and metabolism are acutely apparent in the brain and heart, where a sudden cessation of blood flow, for example following an embolism, acutely manifests as stroke or myocardial infarction. Even in more resilient tissues such as skeletal muscle, a short-term mismatch reduces muscle performance and exercise tolerance, and can cause intermittent claudication. In the longer-term, a microvascular-metabolic mismatch in skeletal muscle reduces insulin-mediated muscle glucose uptake, leading to disturbances in whole-body metabolic homeostasis. While the notion that capillary blood flow is fine-tuned to meet cellular metabolism is well accepted, the mechanisms that control this function and where and how different parts of the vascular tree contribute to capillary blood flow regulation remain poorly understood. Here, we discuss the emerging evidence implicating pericytes, mural cells that surround capillaries, as key mediators that match tissue metabolic demand with adequate capillary blood flow in a number of organs, including skeletal muscle.

Biofabrication of thick vascularized neo-pedicle flaps for reconstructive surgery

Wound chronicity due to intrinsic and extrinsic factors perturbs adequate lesion closure and reestablishment of the protective skin barrier. Immediate and proper care of chronic wounds is necessary for a swift recovery and a reduction of patient vulnerability to infection. Advanced therapies supplemented with standard wound care procedures have been clinically implemented to restore aberrant tissue; however, these treatments are ineffective if local vasculature is too compromised to support minimally-invasive strategies. Autologous "flaps", which are tissues equipped with their own hierarchical vascular supply, can be harvested from one region of the patient and transplanted to the wound where it is reperfused upon microsurgical anastomosis to appropriate recipient vessels. Despite the success of autologous flap transfer, these procedures are extremely invasive, incur obligatory donor-site morbidity, and require sufficient donor-tissue availability, microsurgical expertise, and specialized equipment. 3D-bioprinting modalities, such as extrusion-based bioprinting, can be used to address the clinical constraints of autologous flap transfer, primarily addressing donor-site morbidity and tissue availability. This advancement in regenerative medicine allows the biofabrication of heterogeneous tissue structures with high shape fidelity and spatial resolution to generate biomimetic constructs with the anatomically-precise geometries of native tissue to ensure tissue-specific function. Yet, meaningful progress toward this clinical application has been limited by the lack of vascularization required to meet the nutrient and oxygen demands of clinically relevant tissue volumes. Thus, various criteria for the fabrication of functional tissues with hierarchical, patent vasculature must be considered when implementing 3D-bioprinting technologies for deep, chronic wounds.

Consensus guidelines for the use and interpretation of angiogenesis assays

The formation of new blood vessels, or angiogenesis, is a complex process that plays important roles in growth and development, tissue and organ regeneration, as well as numerous pathological conditions. Angiogenesis undergoes multiple discrete steps that can be individually evaluated and quantified by a large number of bioassays. These independent assessments hold advantages but also have limitations. This article describes in vivo, ex vivo, and in vitro bioassays that are available for the evaluation of angiogenesis and highlights critical aspects that are relevant for their execution and proper interpretation. As such, this collaborative work is the first edition of consensus guidelines on angiogenesis bioassays to serve for current and future reference.

HAMSCs/HBMSCs coculture system ameliorates osteogenesis and angiogenesis against glucolipotoxicity

Osteoporosis and vascular lesions induced by glucolipotoxicity are common complications of diabetes mellitus (DM). In order to deal with these complications, we designed a new therapeutic strategy, i.e. coculture system containing human amnion-derived mesenchymal stem cells (HAMSCs) and human bone marrow mesenchymal stem cells (HBMSCs). Two in vitro coculture models, transwell and mixed cocultures, were proposed for 7 days with variable HAMSCs: HBMSCs ratios. Then, supernatant from each coculture was used to reverse the deficiency of HBMSCs and human umbilical vein endothelial cells (HUVECs) impaired by high glucose and palmitic acid (GP). We found that glucolipotoxicity caused by GP remarkably inhibited cell proliferation, osteogenic differentiation and superoxide dismutase (SOD) activity, as well as induced the reactive oxygen species (ROS) level in HBMSCs. Meanwhile, glucolipotoxicity suppressed cell proliferation, tube formation capacity and angiogenic potential of HUVECs. Though, HAMSCs/HBMSCs coculture system reduced HBMSCs dysfunction by antioxidant properties and promoted angiogenesis in HUVECs. The mixed HAMSCs/HBMSCs coculture at the optimal ratio of 3/1 showed significantly greater cell proliferation, antioxidant properties, osteogenic and angiogenic differentiation than HBMSCs or HUVECs alone. In conclusion, the current coculture system of HAMSCs/HBMSCs can be a potential therapeutic material for advancing bone and vascular regeneration against DM-induced glucolipotoxicity.

Evaluation and Characterization of Endothelial Cell Invasion and Sprouting Behavior

Here, we describe highly reproducible methods to investigate human EC invasion and sprouting behavior in 3D collagen matrices. Two assay models are presented whereby ECs are induced to sprout from a monolayer surface or from aggregated ECs suspended within a collagen gel matrix. In each case, the assays are performed using serum-free defined media containing a combination of five growth factors (Factors): FGF-2, SCF, IL-3, SDF-1α, and insulin. In both models, marked EC sprouting occurs with leading EC tip cells over a 12-24 h period. To illustrate their utility, we present data showing the influence of various pharmacologic inhibitors directed to membrane-type matrix metalloproteinases (MT-MMPs), protein kinase C alpha (PKCα), Src family kinases, and Notch-dependent signaling. Marked inhibition of sprouting is observed after blockade of MT-MMPs and PKCα, while strong increases in sprouting and EC tip cell number is observed following blockade of Src kinases, Notch signaling or both. Interestingly, the increased sprouting behavior observed following Src or Notch blockade directly correlates with a loss in the ability of ECs to form lumens. These defined in vitro assay models allow for a genetic and signaling dissection of EC tip cells vs. lumen forming ECs, which are both necessary for the formation of branching networks of tubes during vascular morphogenic events.

Molecular Regulation of Sprouting Angiogenesis

The term angiogenesis arose in the 18th century. Several studies over the next 100 years laid the groundwork for initial studies performed by the Folkman laboratory, which were at first met with some opposition. Once overcome, the angiogenesis field has flourished due to studies on tumor angiogenesis and various developmental models that can be genetically manipulated, including mice and zebrafish. In addition, new discoveries have been aided by the ability to isolate primary endothelial cells, which has allowed dissection of various steps within angiogenesis. This review will summarize the molecular events that control angiogenesis downstream of biochemical factors such as growth factors, cytokines, chemokines, hypoxia-inducible factors (HIFs), and lipids. These and other stimuli have been linked to regulation of junctional molecules and cell surface receptors. In addition, the contribution of cytoskeletal elements and regulatory proteins has revealed an intricate role for mobilization of actin, microtubules, and intermediate filaments in response to cues that activate the endothelium. Activating stimuli also affect various focal adhesion proteins, scaffold proteins, intracellular kinases, and second messengers. Finally, metalloproteinases, which facilitate matrix degradation and the formation of new blood vessels, are discussed, along with our knowledge of crosstalk between the various subclasses of these molecules throughout the text. Compr Physiol 8:153-235, 2018.

Src- and Fyn-dependent apical membrane trafficking events control endothelial lumen formation during vascular tube morphogenesis

Here we examine the question of how endothelial cells (ECs) develop their apical membrane surface domain during lumen and tube formation. We demonstrate marked apical membrane targeting of activated Src kinases to this apical domain during early and late stages of this process. Immunostaining for phosphotyrosine or phospho-Src reveals apical membrane staining in intracellular vacuoles initially. This is then followed by vacuole to vacuole fusion events to generate an apical luminal membrane, which is similarly decorated with activated phospho-Src kinases. Functional blockade of Src kinases completely blocks EC lumen and tube formation, whether this occurs during vasculogenic tube assembly or angiogenic sprouting events. Multiple Src kinases participate in this apical membrane formation process and siRNA suppression of Src, Fyn and Yes, but not Lyn, blocks EC lumen formation. We also demonstrate strong apical targeting of Src-GFP and Fyn-GFP fusion proteins and increasing their expression enhances lumen formation. Finally, we show that Src- and Fyn-associated vacuoles track and fuse along a subapically polarized microtubule cytoskeleton, which is highly acetylated. These vacuoles generate the apical luminal membrane in a stereotypically polarized, perinuclear position. Overall, our study identifies a critical role for Src kinases in creating and decorating the EC apical membrane surface during early and late stages of lumen and tube formation, a central event in the molecular control of vascular morphogenesis.

Human lung fibroblast-derived matrix facilitates vascular morphogenesis in 3D environment and enhances skin wound healing

Extracellular matrix (ECM) is crucial to many aspects of vascular morphogenesis and maintenance of vasculature function. Currently the recapitulation of angiogenic ECM microenvironment is still challenging, due mainly to its diverse components and complex organization. Here we investigate the angiogenic potential of human lung fibroblast-derived matrix (hFDM) in creating a three-dimensional (3D) vascular construct. hFDM was obtained via decellularization of in vitro cultured human lung fibroblasts and analyzed via immunofluorescence staining and ELISA, which detect multiple ECM macromolecules and angiogenic growth factors (GFs). Human umbilical vein endothelial cells (HUVECs) morphology was more elongated and better proliferative on hFDM than on gelatin-coated substrate. To prepare 3D construct, hFDM is collected, quantitatively analyzed, and incorporated in collagen hydrogel (Col) with HUVECs. Capillary-like structure (CLS) formation at 7day was significantly better with the groups containing higher doses of hFDM compared to the Col group (control). Moreover, the group (Col/hFDM/GFs) with both hFDM and angiogenic GFs (VEGF, bFGF, SDF-1) showed the synergistic activity on CLS formation and found much larger capillary lumen diameters with time. Further analysis of hFDM via angiogenesis antibody array kit reveals abundant biochemical cues, such as angiogenesis-related cytokines, GFs, and proteolytic enzymes. Significantly up-regulated expression of VE-cadherin and ECM-specific integrin subunits was also noticed in Col/hFDM/GFs. In addition, transplantation of Col/hFMD/GFs with HUVECs in skin wound model presents more effective re-epithelialization, many regenerated hair follicles, better transplanted cells viability, and advanced neovascularization. We believe that current system is a very promising platform for 3D vasculature construction in vitro and for cell delivery toward therapeutic applications in vivo.

STATEMENT OF SIGNIFICANCE

Functional 3D vasculature construction in vitro is still challenging due to the difficulty of recapitulating the complex angiogenic extracellular matrix (ECM) environment. Herein, we present a simple and practical method to create an angiogenic 3D environment via incorporation of human lung fibroblast-derived matrix (hFDM) into collagen hydrogel. We found that hFDM offers a significantly improved angiogenic microenvironment for HUVECs on 2D substrates and in 3D construct. A synergistic effect of hFDM and angiogenic growth factors has been well confirmed in 3D condition. The prevascularized 3D collagen constructs also facilitate skin wound healing. We believe that current system should be a convenient and powerful platform in engineering 3D vasculature in vitro, and in delivering cells for therapeutic purposes in vivo.

Uridine adenosine tetraphosphate acts as a proangiogenic factor in vitro through purinergic P2Y receptors

Uridine adenosine tetraphosphate (Up4A), a dinucleotide, exerts vascular influence via purinergic receptors (PR). We investigated the effects of Up4A on angiogenesis and the putative PR involved. Tubule formation assay was performed in a three-dimensional system, in which human endothelial cells were cocultured with pericytes with various Up4A concentrations for 5 days. Expression of PR subtypes and angiogenic factors was assessed in human endothelial cells with and without P2Y6R antagonist. No difference in initial tubule formation was detected between Up4A stimulation and control conditions at day 2 In contrast, a significant increase in vascular density in response to Up4A was observed at day 5 Up4A at an optimal concentration of 5 μM promoted total tubule length, number of tubules, and number of junctions, all of which were inhibited by the P2Y6R antagonist MRS2578. Higher concentrations of Up4A (10 μM) had no effects on angiogenesis parameters. Up4A increased mRNA level of P2YRs (P2Y2R, P2Y4R, and P2Y6R) but not P2XR (P2X4R and P2X7R) or P1R (A2AR and A2BR), while Up4A upregulated VEGFA and ANGPT1, but not VEGFR2, ANGPT2, Tie1, and Tie2. In addition, Up4A increased VEGFA protein levels. Transcriptional upregulation of P2YRs by Up4A was inhibited by MRS2578. In conclusion, Up4A is functionally capable of promoting tubule formation in an in vitro coculture system, which is likely mediated by pyrimidine-favored P2YRs but not P2XRs or P1Rs, and involves upregulation of angiogenic factors.

Dual Role of Mesenchymal Stem Cells Allows for Microvascularized Bone Tissue-Like Environments in PEG Hydrogels

In vitro engineered tissues which recapitulate functional and morphological properties of bone marrow and bone tissue will be desirable to study bone regeneration under fully controlled conditions. Among the key players in the initial phase of bone regeneration are mesenchymal stem cells (MSCs) and endothelial cells (ECs) that are in close contact in many tissues. Additionally, the generation of tissue constructs for in vivo transplantations has included the use of ECs since insufficient vascularization is one of the bottlenecks in (bone) tissue engineering. Here, 3D cocultures of human bone marrow derived MSCs (hBM-MSCs) and human umbilical vein endothelial cells (HUVECs) in synthetic biomimetic poly(ethylene glycol) (PEG)-based matrices are directed toward vascularized bone mimicking tissue constructs. In this environment, bone morphogenetic protein-2 (BMP-2) or fibroblast growth factor-2 (FGF-2) promotes the formation of vascular networks. However, while osteogenic differentiation is achieved with BMP-2, the treatment with FGF-2 suppressed osteogenic differentiation. Thus, this study shows that cocultures of hBM-MSCs and HUVECs in biological inert PEG matrices can be directed toward bone and bone marrow-like 3D tissue constructs.

Cdc42 and k-Ras Control Endothelial Tubulogenesis through Apical Membrane and Cytoskeletal Polarization: Novel Stimulatory Roles for GTPase Effectors, the Small GTPases, Rac2 and Rap1b, and Inhibitory Influence of Arhgap31 and Rasa1

A critical and understudied property of endothelial cells is their ability to form lumens and tube networks. Although considerable information has been obtained concerning these issues, including the role of Cdc42 and Rac1 and their effectors such as Pak2, Pak4, Par6b, and co-regulators such as integrins, MT1-MMP and Par3; many key questions remain that are necessary to elucidate molecular and signaling requirements for this fundamental process. In this work, we identify new small GTPase regulators of EC tubulogenesis including k-Ras, Rac2 and Rap1b that act in conjunction with Cdc42 as well as the key downstream effectors, IQGAP1, MRCKβ, beta-Pix, GIT1, and Rasip1 (which can assemble into multiprotein complexes with key regulators including α2β1 integrin and MT1-MMP). In addition, we identify the negative regulators, Arhgap31 (by inactivating Cdc42 and Rac) and Rasa1 (by inactivating k-Ras) and the positive regulator, Arhgap29 (by inactivating RhoA) which play a major functional role during the EC tubulogenic process. Human EC siRNA suppression or mouse knockout of Rasip1 leads to identical phenotypes where ECs form extensive cord networks, but cannot generate lumens or tubes. Essential roles for these molecules during EC tubulogenesis include; i) establishment of asymmetric EC cytoskeletal polarization (subapical distribution of acetylated tubulin and basal membrane distribution of F-actin); and ii) directed membrane trafficking of pinocytic vacuoles or other intracellular vesicles along acetylated tubulin tracks to the developing apical membrane surface. Cdc42 co-localizes subapically with acetylated tubulin, while Rac1 and k-Ras strongly label vacuole/ vesicle membranes which accumulate and fuse together in a polarized, perinuclear manner. We observe polarized apical membrane and subapical accumulation of key GTPases and effectors regulating EC lumen formation including Cdc42, Rac1, Rac2, k-Ras, Rap1b, activated c-Raf and Rasip1 to control EC tube network assembly. Overall, this work defines novel key regulators and their functional roles during human EC tubulogenesis.

Characterizing the angiogenic activity of patients with single ventricle physiology and aortopulmonary collateral vessels

OBJECTIVES

Patients with single ventricle congenital heart disease often form aortopulmonary collateral vessels via an unclear mechanism. To gain insights into the pathogenesis of aortopulmonary collateral vessels, we correlated angiogenic factor levels with in vitro activity and angiographic aortopulmonary collateral assessment and examined whether patients with single ventricle physiology have increased angiogenic factors that can stimulate endothelial cell sprouting in vitro.

METHODS

In patients with single ventricle physiology (n = 27) and biventricular acyanotic control patients (n = 21), hypoxia-inducible angiogenic factor levels were measured in femoral venous and arterial plasma at cardiac catheterization. To assess plasma angiogenic activity, we used a 3-dimensional in vitro cell sprouting assay that recapitulates angiogenic sprouting. Aortopulmonary collateral angiograms were graded using a 4-point scale.

RESULTS

Compared with controls, patients with single ventricle physiology had increased vascular endothelial growth factor (artery: 58.7 ± 1.2 pg/mL vs 35.3 ± 1.1 pg/mL, P < .01; vein: 34.8 ± 1.1 pg/mL vs 21 ± 1.2 pg/mL, P < .03), stromal-derived factor 1-alpha (artery: 1901.6 ± 1.1 pg/mL vs 1542.6 ± 1.1 pg/mL, P < .03; vein: 2092.8 pg/mL ± 1.1 vs 1752.9 ± 1.1 pg/mL, P < .02), and increased arterial soluble fms-like tyrosine kinase-1, a regulatory vascular endothelial growth factor receptor (612.3 ± 1.2 pg/mL vs 243.1 ± 1.2 pg/mL, P < .003). Plasma factors and sprout formation correlated poorly with aortopulmonary collateral severity.

CONCLUSIONS

We are the first to correlate plasma angiogenic factor levels with angiography and in vitro angiogenic activity in patients with single ventricle disease with aortopulmonary collaterals. Patients with single ventricle disease have increased stromal-derived factor 1-alpha and soluble fms-like tyrosine kinase-1, and their roles in aortopulmonary collateral formation require further investigation. Plasma factors and angiogenic activity correlate poorly with aortopulmonary collateral severity in patients with single ventricles, suggesting complex mechanisms of angiogenesis.

Investigating human vascular tube morphogenesis and maturation using endothelial cell-pericyte co-cultures and a doxycycline-inducible genetic system in 3D extracellular matrices

Considerable progress has occurred toward our understanding of the molecular basis for vascular morphogenesis, maturation, and stabilization. A major reason for this progress has been the development of novel in vitro systems to investigate these processes in 3D extracellular matrices. In this chapter, we present models of human endothelial cell (EC) tube formation and EC-pericyte tube co-assembly using serum-free defined conditions in 3D collagen matrices. We utilize both human venous and arterial ECs and show that both cell types readily form tubes and induce pericyte recruitment and both ECs and pericytes work together to remodel the extracellular matrix environment by assembling the vascular basement membrane, a key step in capillary tube network maturation and stabilization. Importantly, we have shown that these events occur under serum-free defined conditions using the hematopoietic stem cell cytokines, SCF, IL-3, and SDF-1α and also including FGF-2. In contrast, the combination of VEGF and FGF-2 fails to support vascular tube morphogenesis or pericyte-induced tube maturation under the same serum-free defined conditions. Furthermore, we present novel assays whereby we have developed both human ECs and pericytes to induce specific genes using a doxycycline-regulated lentiviral system. In this manner, we can upregulate the expression of wild-type or mutant gene products at any stage of vascular morphogenesis or maturation in 3D matrices. These in vitro experimental approaches will continue to identify key molecular requirements and signaling pathways that control fundamental events in tissue vascularization under normal or pathologic conditions. Furthermore, these models will provide new insights into the development of novel disease therapeutic approaches where vascularization is an important pathogenic component and create new ways to assemble capillary tube networks with associated pericytes for tissue engineering applications.

Amniotic Mesenchymal Stem Cells Can Enhance Angiogenic Capacity via MMPs In Vitro and In Vivo

The aim of this study was to evaluate the angiogenic capacity and proteolytic mechanism of coculture using human amniotic mesenchymal stem cells (hAMSCs) with human umbilical vein endothelial cells (HUVECs) in vivo and in vitro by comparing to those of coculture using bone marrow mesenchymal stem cells with HUVEC. For the in vivo experiment, cells (HUVEC-monoculture, HUVEC-hAMSC coculture, and HUVEC-BMMSC coculture) were seeded in fibrin gels and injected subcutaneously in nude mice. The samples were collected on days 7 and 14 and histologically analyzed by H&E and CD31 staining. CD31-positive staining percentage and vessel-like structure (VLS) density were evaluated as quantitative parameters for angiogenesis. The increases of CD31-positive staining area and VLS density in both HUVEC-hAMSC group and HUVEC-BMMSC group were found between two time points, while obvious decline of those was observed in HUVEC-only group. For the in vitro experiment, we utilized the same 3D culture model to investigate the proteolytic mechanism related to capillary formation. Intensive vascular networks formed by HUVECs were associated with hAMSCs or BMMSCs and related to MMP2 and MMP9. In conclusion, hAMSCs shared similar capacity and proteolytic mechanism with BMMSCs on neovascularization.

Functional Effects of a Tissue-Engineered Cardiac Patch From Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes in a Rat Infarct Model

A tissue-engineered cardiac patch provides a method to deliver cardiomyoctes to the injured myocardium with high cell retention and large, controlled infarct coverage, enhancing the ability of cells to limit remodeling after infarction. The patch environment can also yield increased survival. In the present study, we sought to assess the efficacy of a cardiac patch made from human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) to engraft and limit left ventricular (LV) remodeling acutely after infarction. Cardiac patches were created from hiPSC-CMs and human pericytes (PCs) entrapped in a fibrin gel and implanted acutely onto athymic rat hearts. hiPSC-CMs not only remained viable after in vivo culture, but also increased in number by as much as twofold, consistent with colocalization of human nuclear antigen, cardiac troponin T, and Ki-67 staining. CM+PC patches led to reduced infarct sizes compared with myocardial infarction-only controls at week 4, and CM+PC patch recipient hearts exhibited greater fractional shortening over all groups at both 1 and 4 weeks after transplantation. However, a decline occurred in fractional shortening for all groups over 4 weeks, and LV thinning was not mitigated. CM+PC patches became vascularized in vivo, and microvessels were more abundant in the host myocardium border zone, suggesting a paracrine mechanism for the improved cardiac function. PCs in a PC-only control patch did not survive 4 weeks in vivo. Our results indicate that cardiac patches containing hiPSC-CMs engraft onto acute infarcts, and the hiPSC-CMs survive, proliferate, and contribute to a reduction in infarct size and improvements in cardiac function.

SIGNIFICANCE

In the present study, a cardiac patch was created from human induced pluripotent stem cell-derived cardiomyocytes and human pericytes entrapped in a fibrin gel, and it was transplanted onto infarcted rat myocardium. It was found that a patch that contained both cardiomyocytes and pericytes survived transplantation and resulted in improved cardiac function and a reduced infarct size compared with controls.

Systems biology of the microvasculature

The vascular network carries blood throughout the body, delivering oxygen to tissues and providing a pathway for communication between distant organs. The network is hierarchical and structured, but also dynamic, especially at the smaller scales. Remodeling of the microvasculature occurs in response to local changes in oxygen, gene expression, cell-cell communication, and chemical and mechanical stimuli from the microenvironment. These local changes occur as a result of physiological processes such as growth and exercise, as well as acute and chronic diseases including stroke, cancer, and diabetes, and pharmacological intervention. While the vasculature is an important therapeutic target in many diseases, drugs designed to inhibit vascular growth have achieved only limited success, and no drug has yet been approved to promote therapeutic vascular remodeling. This highlights the challenges involved in identifying appropriate therapeutic targets in a system as complex as the vasculature. Systems biology approaches provide a means to bridge current understanding of the vascular system, from detailed signaling dynamics measured in vitro and pre-clinical animal models of vascular disease, to a more complete picture of vascular regulation in vivo. This will translate to an improved ability to identify multi-component biomarkers for diagnosis, prognosis, and monitoring of therapy that are easy to measure in vivo, as well as better drug targets for specific disease states. In this review, we summarize systems biology approaches that have advanced our understanding of vascular function and dysfunction in vivo, with a focus on computational modeling.

Molecular control of capillary morphogenesis and maturation by recognition and remodeling of the extracellular matrix: functional roles of endothelial cells and pericytes in health and disease

This review addresses fundamental mechanisms underlying how capillaries form in three-dimensional extracellular matrices and how endothelial cells (ECs) and pericytes co-assemble to form capillary networks. In addition to playing a critical role in supplying oxygen and nutrients to tissues, recent work suggests that blood vessels supply important signals to facilitate tissue development. Here, we hypothesize that another major function of capillaries is to supply signals to suppress major disease mechanisms including inflammation, infection, thrombosis, hemorrhage, edema, ischemic injury, fibrosis, autoimmune disease and tumor growth/progression. Capillary dysfunction plays a key pathogenic role in many human diseases, and thus, this suppressing function may be attenuated and central toward the initiation and progression of disease. We describe how capillaries form through creation of EC-lined tube networks and vascular guidance tunnels in 3D extracellular matrices. Pericytes recruit to the abluminal EC tube surface within these tunnel spaces, and work together to assemble the vascular basement membrane matrix. These processes occur under serum-free conditions in 3D collagen or fibrin matrices and in response to five key growth factors which are stem cell factor, interleukin-3, stromal-derived factor-1α, fibroblast growth factor-2 and insulin. In addition, we identified a key role for EC-derived platelet-derived growth factor-BB and heparin-binding epidermal growth factor in pericyte recruitment and proliferation to promote EC-pericyte tube co-assembly and vascular basement membrane matrix deposition. A molecular understanding of capillary morphogenesis and maturation should lead to novel therapeutic strategies to repair capillary dysfunction in major human disease contexts including cancer and diabetes.

Vasculogenesis and Angiogenesis in Modular Collagen-Fibrin Microtissues

The process of new blood vessel formation is critical in tissue development, remodeling and regeneration. Modular tissue engineering approaches have been developed to enable the bottom-up assembly of more complex tissues, including vascular networks. In this study, collagen-fibrin composite microbeads (100-300 μm in diameter) were fabricated using a water-in-oil emulsion technique. Human endothelial cells and human fibroblasts were embedded directly in the microbead matrix at the time of fabrication. Microbead populations were characterized and cultured for 14 days either as free-floating populations or embedded in a surrounding fibrin gel. The collagen-fibrin matrix efficiently entrapped cells and supported their viability and spreading. By 7 days in culture, endothelial cell networks were evident within microbeads, and these structures became more prominent by day 14. Fibroblasts co-localized with endothelial cells, suggesting a pericyte-like function, and laminin deposition indicated maturation of the vessel networks over time. Microbeads embedded in a fibrin gel immediately after fabrication showed the emergence of cells and the coalescence of vessel structures in the surrounding matrix by day 7. By day 14, inosculation of neighboring cords and prominent vessel structures were observed. Microbeads pre-cultured for 7 days prior to embedding in fibrin gave rise to vessel networks that emanated radially from the microbead by day 7, and developed into connected networks by day 14. Lumen formation in endothelial cell networks was confirmed using confocal sectioning. These data show that collagen-fibrin composite microbeads support vascular network formation. Microbeads embedded directly after fabrication emulated the process of vasculogenesis, while the branching and joining of vessels from pre-cultured microbeads resembled angiogenesis. This modular microtissue system has utility in studying the processes involved in new vessel formation, and may be developed into a therapy for the treatment of ischemic conditions.